1. Field of the Invention
The present invention relates to honeycomb structures and exhaust gas treating apparatuses.
2. Description of the Related Art
A honeycomb structure is conventionally used in exhaust gas treating apparatuses for treating NOx and the like in automobile emissions, as disclosed in Japanese Laid-Open Patent Application No. 2006-183477.
The honeycomb structure typically includes plural cells (through holes) extending in a longitudinal direction of the structure from one end surface to the other. These cells are partitioned by cell walls.
The cell walls of the honeycomb structure are typically made of ceramic, such as cordierite. On the cell walls, a NOx adsorbing material and an ammonia adsorbing material are disposed. A layer of the NOx adsorbing material, which may include ceria or the like, supports a noble metal catalyst, such as platinum. The ammonia adsorbing material may include zeolite or the like.
As automobile exhaust gases or the like pass through such a honeycomb structure, NOx in the exhaust gas is adsorbed onto the NOx adsorption layer when the exhaust gas is in an oxidative atmosphere (such as during a normal operation of a diesel engine). The adsorbed NOx is reduced on the catalyst when the exhaust gas is in a reductive atmosphere (such as in a spike period of the diesel engine) into ammonia, which is adsorbed onto the ammonia adsorption layer. When the exhaust gas returns to an oxidative atmosphere, the NOx is reduced using the ammonia. Upon running out of the adsorbed ammonia, NOx is adsorbed onto the NOx adsorption layer.
By thus passing the exhaust gas through the honeycomb structure, NOx in the exhaust gas can be treated. Meanwhile, a honeycomb structure composed of inorganic particles, inorganic fibers, and an inorganic binder is described in WO2005/063653.
The contents of the aforementioned documents Japanese Laid-Open Patent Application No. 2006-183477 and WO2005/063653 are hereby incorporated by reference herein in their entirety.